Hi,
with Your Spin Control You have a message handler:
OnDeltapos(NMHDR* pNMHDR, LRESULT* pResult)
Class wizard defines a variable in it:
NM_UPDOWN* pNMUpDown = (NM_UPDOWN*)pNMHDR;
The numbers are changed with the value
pNMUpDown->iDelta
To swap it You only have to multiplicate it with -1.
BTW: the direction of the spin button is exactly the same as it is with scrollbars. So, lots of people (including me) would expect the standard behavior...

HTH
Rudolf